This invention concerns a surface acoustic wave (SAW) filter comprising a piezoelectric substrate having send and receive transducers applied to its surface. The transducers are formed by a multiplicity of electrodes arranged in an interlocking-finger design that are connected in two half-groups either to one terminal for a signal with a first phase relationship, or to another terminal for a signal with a second phase relationship, while a meandering ground line is led between the two half-groups.
SAW (Surface Acoustic Wave) filters are generally known to be electrical filters operating with elastic compressional (acoustic) waves that propogate on the surfaces of piezoelectric crystals. However, as is described, for example, in the article "Mikrowellenakustik" (Microwave Acoustics) in Technische Rundschau, No. 16, Apr. 18, 1978, pp. 17, such SAW filters can operate with frequencies that are well above the strictly acoustical range. The core element of a typical SAW filter consists of a piezoelectric substrate having both a send and a receive transducer applied to its surface. These two transducers are comprised of electrodes arranged in an interlocking-finger design, in which the distance between two successive electrodes as well as the width of the electrodes are constant.
Such SAW filters operate in a manner that, if an electrical input signal is applied to the send transducer from a signal source, a periodic electrical field is generated at the send transducer and, due to the effect of the piezoelectric coupling, the input signal is converted into a surface acoustic wave that propagates along the piezoelectric substrate. At the receive transducer, the surface acoustic wave is converted into an electrical output signal which is applied to an external load. Send and receive transducers thus form a converter with a characteristic transfer function. In practice, SAW filters and constructed to provide very distinctive filter characteristics.
Presently, with simple transducer types the surface waves are propagated in the two directions, resulting in losses. Furthermore, it is not possible to match simple transducers to the source impedance. Therefore, directional transducers are used that operate as three-phase transducers or as group transducers. Three-phase transducers have a multilayer structure; therefore, their production is very complex.
The group transducers, on the other hand, have a planar construction and can be manufactured considerably easier.
Known group transducers have several groups of electrodes each consisting of 2n electrodes of which n successive electrodes form half a group of one phase relationship and the remaining n successive electrodes form half a group of another phase relationship while a meandering ground line is led between each two group halves. Furthermore, additional grounded electrode fingers are threaded between the n electrodes. As a result, a source impedance match for a filter with several groups can only be achieved by means of phase shifters. Such filters, however, prove to be of disadvantage with regard to the undesirable side frequency maxima. To avoid this disadvantage, transducers are known in which the finger electrodes have varying lengths. See, for example, the Hewlett-Packard Journal, Dec. 1981, page 5, (FIG. 3); "IEEE Group on Sonics and Ultrasonics" Ultrasonics Symposium, Sept. 22-24, 1975, pp. 317-321; and 6th European Microwave Conference 1976, Rome, Sept. 14 to 17, 1976, AEI, pp. 267-271.
Side frequency maxima can be suppressed by these measures; however, relatively complicated phase shifters must be provided.